The invention relates generally to the manufacture and test of optical transceivers including a modulated laser, and more specifically to an automated system and method for biasing and driving a modulated laser, such as an electro-absorptive modulated laser in an optical transceiver, in order to achieve desired operational characteristics, and for bit error rate screening of optical transmitters and receivers.
Communication devices using optical transmission technologies have been increasingly employed in recent years to satisfy the demand for large capacity digital transmission in communication networks, such as public telecommunications networks. In its basic form, an optical communication system includes a modulated laser, an optical fiber link and an optical receiver. The laser receives as an input an electrical signal in the form of a high speed serial bit stream which may include both encoded clock and data information. In response to the received serial bit stream, the laser output intensity is modulated to generate a modulated light signal which is propagated onto the optical fiber link. The light then passes through the fiber to an optical receiver which transforms the received modulated light signal into an electrical serial bit stream. The electrical signal is then amplified, conditioned, and fed to another circuit which performs clock and data recovery.
One challenge faced by manufacturers of optical communication equipment results from the fact that operational characteristics of individual lasers vary to such an extent that each laser and associated modulator must be biased individually to achieve desired power output and modulation characteristics. Existing approaches to this problem have included the manual entry of values which control the operational characteristics of the modulated laser into a non-volatile memory, such as a programmable read only memory (PROM). Such manually entered values are typically based on values provided in data sheets for each individual laser obtained from the manufacturer of the respective laser. This type of a manual process, however, is time consuming and error-prone, and can result in unacceptably high rates of mis-entry, causing operational problems. Additionally, the laser data sheet values used to determine control values for the laser modulators are not based on operational characteristics of the specific laser in the device in which it will be used. Accordingly, control values, such as a laser power bias current control vale and EA modulator bias control values, may not be optimally established in existing systems where a laser""s in-circuit operational characteristics differ significantly from the characteristics specified by the manufacturer of the laser. For this reason, existing approaches, whether manual or automated, may not optimally adjust the parameters of the laser and/or receiver to the surrounding circuit.
Another problem in the manufacturing and testing of optical communication equipment relates to the testing of the optical transmitter and receiver functions to determine whether an optical transceiver meets predetermined bit error rate criteria. Where a device is required to operate with a bit error rate (BER) below some specified minimum, testing is desirable to ensure that the desired BER will be met when the transceiver is deployed in a telecommunication network. For example, an optical transceiver board may be tested to determine whether both the transmitter and receiver functions introduce errors at a rate below a threshold bit error rate. In the case where the threshold error rate is extremely low with regard to the transmission rate of the device, such testing may require weeks for each device. Such delays are highly undesirable in a manufacturing process. For example, the testing of a transceiver for a device supporting SONET OC-48 to determine whether it would meet a BER specification of less than 10xe2x88x9215 bits per second would require several weeks of continuous test transmissions to get a reliable estimate of the BER. Such long testing periods are costly and delay the shipments of product by the optical transceiver manufacturer. Furthermore, transceivers are often intended for use over long distance, optically amplified links. In such links, as the transmitted optical signal passes through numerous amplifiers, amplifier noise (Amplified Spontaneous Emission or ASE) accumulates. Accordingly, a performance metric for such amplified, long distance links may be thought of as the ability to maintain a maximum BER as a function of the optical signal-to-noise-ratio (OSNR) at the receiver. It would be greatly beneficial to have an automated system for characterizing transceivers during the manufacturing process as a function of OSNR that does not require multiple amplifiers, due to the costs and maintenance constraints associated with their use.
For these reasons it would be desirable to have an efficient and reliable automated method and apparatus for biasing a modulated laser used in an optical transceiver. The system should reduce the risk of errors occurring during manual entry of control values, and provide a more accurate determination of control values reflecting the actual circuit in which the modulated laser will operate. Additionally, the testing system should permit bit error rate testing down to very low levels using tests that do not involve prolonged test times.
In accordance with principles of the invention, a system for biasing and testing an optical transceiver is disclosed. In a first aspect of the disclosed system, biasing of a modulated laser is performed under the control of a test control system. The test control system may include a computer or test controller, and the steps employed in the determination of the desired bias values are performed in response to a software program executing on the test controller. The test controller is communicably coupled to the module under test, for example, through a digital interface, such as a serial RS-232 communications link. The test controller may further be communicably connected to an optical power level monitor and other testing devices through a communication bus, such as the General Purpose Interface Bus (GPIB). Through the digital interface to the module under test, or by way of the other testing devices coupled to the test controller through the communication bus, the test controller can modify operational bias parameters of the modulated laser, in order to provide a desired extinction ratio at a desired power output level. The operational parameters may include, for example, the laser power bias current, an electro-absorption (EA) modulator bias offset voltage and an electro-absorption (EA) laser modulation depth. During operation of the disclosed system, the test controller configures the test system, establishes the laser bias current necessary to achieve a desired laser on state output power, determines a desired EA modulator bias current necessary to achieve a desired off state laser power and adjusts a modulation depth control value to achieve the desired extinction ratio.
In another aspect of the disclosed system, the test controller determines whether the optical transmitter and the optical receiver within the transceiver each exhibit an acceptable bit error rate (BER), as a function of either received optical power or received OSNR. To perform the bit error rate testing, the test controller uses a variable optical attenuator, in combination with an erbium doped optical fiber amplifier (EDFA) to control an optical signal to noise ratio (OSNR) in a signal terminating at either a reference receiver or the receiver of the optical transceiver in the module under test. A bit error rate tester is coupled to a pre-qualified transceiver module which includes a reference transmitter and the reference receiver. The reference receiver and transmitter are employed in conjunction with the transmitter and receiver of the module under test respectively to measure a number of bit error rates corresponding to controlled optical signal to noise ratios. More specifically, the generated optical signal to noise ratios are gradually increased, resulting in lower measured bit error rates. The disclosed system samples the bit error rates for a number of relatively low signal to noise ratios, and projects an optical signal to noise ratio that would be required to obtain a predetermined, relatively low bit error rate. In this manner, the disclosed system avoids the need to actually perform prolonged bit error rate measurements to verify very low bit error rates. If the projected optical signal to noise ratio for the low bit error rate is above a predetermined maximum optical signal to noise ratio, then an indication is provided that the optical transmitter or receiver being tested within the module under test has failed the test. In an alternative embodiment, the transmitted Q over the simulated amplifier link created using the generated optical signal to noise ratio can be measured as an indicator or metric of the transmitted BER quality.
Thus there is provided an efficient and reliable automated method and apparatus for biasing and testing an optical transceiver. The disclosed system reduces the risk of errors occurring during manual entry of laser bias and modulator control values and provides a more accurate determination of control values desired in the actual module in which the modulated laser well be employed. Additionally, the disclosed system allows bit error rate qualification down to very low bit error rates using tests that greatly reduce the actual test times required for each device.